Field of the Invention
The primary starting material for commercially employed steroids are plant steroids. Since the plant steroids differ as to substituents on the ring and particularly as to the substituent at the C-17 position, substantial modification of the plant steroid is required. In addition, because one is concerned with a product which is susceptible to fluctuation and availability, there have been substantial efforts to find alternative means for preparing steroidal compounds.
In synthesizing steroids from small readily available organic fragments, one must be concerned with the complex structure of the polycyclic steroid, as well as the particular geometry of the substituents and the ring fusions. In synthesizing a steroid, it is therefore necessary that one introduce the various substituents having the desired geometry and in preparing the rings, provide a means for obtaining the ring fusions having the specified geometry. Since the synthesis will invariably involve a number of steps, it is necessary during the synthetic sequence to insure that subsequent steps do not affect prior conformations which have been introduced so as to provide desired geometry. In addition, where catalyzed cyclization of a polyunsaturated compound is involved there must be concern about the effect of various substituents on the carbon atoms which will ultimately be annular carbon atoms. Since the cyclized compound will be more compact then the acyclic compound, non-bonded steric interactions can be expected to deleteriously affect the course of the cyclization. Where heteroatoms are present on the polyunsaturated chain, the inductive effect of the heteroatoms, their interaction with acid, and their ultimate geometry must be taken into account. In addition, the choice of the polyunsaturated compound must allow for cyclization to the steroidal structure without interruption, so that the tetracyclic compound is formed, rather than a lower order polycyclic compound.